The present invention relates generally to the field of fabricating elements in one substrate, which elements are received by openings in a receiving substrate and also to apparatuses constituting or containing these elements.
There are many examples of large arrays of functional components which can provide, produce or detect electromagnetic signals or chemicals or other characteristics. An example of such a large array is that of a display where many pixels or sub-pixels are formed with an array of electronic elements. For example, an active matrix liquid crystal display includes an array of many pixels or sub-pixels which are fabricated on amorphous silicon or polysilicon substrates which are large. As is well known in the art, it is difficult to produce a completely flawless active matrix liquid crystal display (LCD), when the display area is large, such as the LCD""s on modem laptop computers. As the display area gets larger and larger, the yield of good displays decreases. This is due to the manner in which these display devices are fabricated.
Silicon VLSI can be used to produce such an array over a silicon wafer""s surface, but silicon wafers are limited in size, limited in conductivity, and not transparent. Further, processing of large areas on silicon wafers can be expensive. Displays which valve the light coming through them need to be transparent. Single crystal silicon can be bonded to a glass substrate and then etched to remove most of the area to achieve transparency, but this is intrinsically wasteful in that, for the sake of maximizing light transmission, the majority of the processed material is discarded and becomes chemical waste. The under-utilization of the precious die area wastes resources, causes greater amounts of chemical waste to be generated in the process, and is generally inefficient and expensive. Another example is photodiode arrays which may be used to collect solar energy. Large arrays of silicon photodiodes with concentrating lenses have been made by sawing wafers and using a pick and place assembly, but thermal dissipation is poor for large elements, and the small elements require too much assembly time.
Alternative approaches to fabricating arrays such as displays include fabricating the desired circuitry in an amorphous or polycrystalline semiconductor layer which has been deposited on a substrate, such as glass or quartz. These approaches avoid the limitations of the size of the available single crystal silicon wafers, and avoid the cost of the single crystal wafers, but require expensive deposition of the semiconductor layer, and they still require processing of the entire large substrate to form the active elements in an array, still resulting in the production of much chemical waste and wasted resources. These processes also limit the choice of the substrate; for example, plastic substrates cannot be used due to the nature of the processes which deposit the semiconductor layers. Furthermore, amorphous or polycrystalline silicon semiconductor elements do not perform as well as those made from single crystal semiconductor material. For displays, as an example, it is often difficult or impossible to form some of the desired circuitry out of the amorphous or polycrystalline semiconductor materials. Thus, high frequency edge drivers may be impossible to form out of these materials. This results in the difficulty and expense of attaching an electrical lead for each and every row and column of an array, such as an active matrix liquid crystal display array.
As noted above, another difficulty with the existing techniques is that the large number of elements in a large array results in a low probability that all of them will work properly and thus the yield of acceptably good arrays from the manufacturing process is low. Furthermore, there is no possibility of testing any of the elements until the assembly is complete, and then any imperfection in the array must be tolerated or the entire array could be discarded or special and expensive techniques must be used to repair it. These problems result from the fact that the various elements in the array are fabricated on the array rather than separately.
It is possible to separately produce elements, such as pixel drivers and then place them where desired on a different and perhaps larger substrate. Prior techniques can be generally divided into two types: deterministic methods or random methods. Deterministic methods, such as pick and place, use a human or robot arm to pick each element and place it into its corresponding location in a different substrate. Pick and place methods place devices generally one at a time, and are generally not applicable to very small or numerous elements such as those needed for large arrays, such as an active matrix liquid crystal display. Random placement techniques are more effective and result in high yields if the elements to be placed have the right shape. U.S. Pat. No. 5,545,291 describes a method which uses random placement. In this method, microstructures are assembled onto a different substrate through fluid transport. This is sometimes referred to as fluidic self assembly. Using this technique, various blocks, each containing a functional component, may be fabricated on one substrate and then separated from that substrate and assembled onto a separate substrate through the fluidic self assembly process.
Thus the process which uses fluidic self assembly typically requires forming elements or blocks, each containing at least one functional component and each having a predetermined shape which is designed to fit into openings in a receiving substrate. Two methods are known in the prior art for forming such elements and are described in U.S. Pat. No. 5,545,291. One such method is shown in FIGS. 1A through 1F which show a cross-sectional view of a process of fabricating a plurality of blocks. The substrate 10 shown in cross-section view in FIG. 1A may be a gallium arsenide wafer. A sacrificial layer 12 is formed on the top of the substrate 10 as shown in FIG. 1B. This sacrificial layer may be formed by CVD (Chemical Vapor Deposition) or sputtering and may be formed from aluminum arsenide. Then an active layer from which the blocks will be created is deposited over the sacrificial layer 12. The active layer 14 is shown applied over the sacrificial layer 12 in FIG. 1C. A masking material is then applied over the active layer 14 and is patterned thereby creating the masks 16 as shown in FIG. 1D. Then an etching process is performed on the structure shown in FIG. 1D to etch away the active layer 14, except where masked, down to the sacrificial layer 12. This results in the structure shown in FIG. 1E in which the blocks 18A and 18B have been formed underneath the masks 16. The shaped blocks in this case include a trapezoidal profile or truncated pyramid shape which may be fabricated by methods of wet etching, plasma etching, ion milling, or reactive ion etching, depending on the materials and the application. The masked layer 16 may then be removed (e.g., by using a photoresist stripper if the masks are formed from developed photoresist) and then the sacrificial layer is preferentially etched (e.g., with a wet hydrofluoric acid etch depending on the materials used for the sacrificial layer relative to the materials used for the blocks). This causes the blocks 18A and 18B to be released. The blocks can then be collected and placed into a slurry such as a fluid for later use in the fluidic self assembly process or in other assembly processes.
FIGS. 2A through 2D show an alternative approach for fabricating elements or blocks which can be later assembled into openings in a receiving substrate. The method shown in FIGS. 2A through 2D begins with the structure of FIG. 1E by taking that structure and filling it, on the sides with the blocks, with a material such as a wax or other filler thereby producing the structure in FIG. 2A which is a cross-sectional view through the blocks and the substrate. A substrate 22, which may be a silicon wafer, may then be applied to the block side of the structure shown in FIG. 2A, which results in the structure shown in the cross-sectional view of FIG. 2B. Typically, the native oxide layer on the substrate 22 is removed from the face of the substrate before applying the substrate to the filler material 20 and the mask material 16. The top of the structure shown in FIG. 2B is then lapped in order to remove the substrate 10. In one embodiment, this top substrate layer is lapped up to 50 microns and then etched away up to the sacrificial layer 12 which acts as an etch stop. In one particular embodiment, this etch may be a hydrogen peroxide and ammonium hydroxide etchant. Following this etch, the sacrificial layer which in one embodiment is an aluminum arsenide layer is removed thereby producing the structure shown in FIG. 2C, which is a cross-sectional view. Then the filler material 20 and the photoresist material 16, which is used as a mask, is dissolved away in order to release the blocks 18B and 18A.
While the foregoing methods allow for the creation of blocks, these methods do not easily produce precisely formed blocks, and particularly precisely formed sidewalls of the blocks.
The present invention provides methods for creating elements of a predetermined shape which are designed to fit into openings in a receiving substrate. In one example of a method of the present invention, a plurality of functional components in a plurality of elements are fabricated in a first substrate. The functional components are disposed on a first face of the first substrate, which includes the first face and a second face. A layer representing a first portion of the first substrate at the second face is removed to leave a second portion of the first substrate. The second portion of the first substrate is etched through a first patterned mask on a surface of the second portion, and the plurality of elements is released from the first substrate. In one particular example of the present invention according to this method, the plurality of elements once released are combined with a fluid to form a slurry which may then be applied over a substrate which includes openings to receive the elements.
In another example of a method according to the present invention, a plurality of functional components and a plurality of elements is fabricated in a first substrate which includes a first face and a second face, wherein the plurality of functional components are disposed on the first face of the substrate. Regions of the first face which are adjacent to the edges of the plurality of components are etched vertically. Regions below the first face which are adjacent to the edges are etched laterally, and the plurality of elements is released from the first substrate. In one particular specific example of this method, the vertical etch is performed with a reactive ion etch, and the lateral etch is an undercutting etch.
According to another example of a method of the present invention, a tungsten layer is deposited on a first face which includes a plurality of functional components in a plurality of elements which are fabricated in the first substrate which includes the first face. A second face of the first substrate is etched to expose at least a portion of the plurality of elements, and then the layer including tungsten is etched. In one particular specific embodiment according to this method, the etching of the tungsten layer releases the plurality of elements from the first substrate.
In another example of a method according to the present invention, a structure may be assembled on a receiving substrate by dispensing the slurry over the receiving substrate which includes a plurality of openings. This slurry includes a fluid and a plurality of shaped elements, where the shaped elements have been fabricated according to one of the methods of the present invention.
Other aspects and methods of the present invention as well as apparatuses formed using these methods are described further below in conjunction with the following figures.